Thunderbolts are probably the worst ammo you can use anyway.
They truly are: Just look up remington thunderbolts on youtube and you'll probably see some serious fouling videos in .22lr. When using the thunderbolts, ya better be sure to spend a good amount of time with the barrel, or it's gonna build up. Infact- I shoot CCI Blazer lead RN ammo a lot, but everytime after the session, I make sure to give the barrel a good cleaning-with solvent and the nylon brush. So I never have the problem.
As with Thunderbolts, beware that they are the worst lead ammo you can put through your pistol/ rifle, especially in high volume.
In this case it's also related to the nature of using plain lead ammo...- ya gotta spend a lot more time scrubbing/ cleaning. If ya don't want that, stick with the copper washed ammo.
Remington rimfire ammo IS the worst you can buy
Bingo!
Don't buy Remington. Stick with par grade ammo to CCI if ya want something that is cleaner and infinitely more reliable. The first time I bought a .22lr pistol, I tried a few brands of ammo. Remington must be some of the dirtiest loose weak priming ammo out there. I've gotten duds in center fire ammo manufactured by Remington!?! That says a lot to me about which brand ammo to stay far away from.